In free space optical communication, an infrared light source transmits a collimated beam to an intended target (e.g., remote control for television and stereo applications). The size of the transmitted beam gets bigger as it propagates along the way. The optical power carried by the beam, however, generally attenuates to some extent, depending on the propagation distance. As a result, the optical intensity (which is the optical power per unit area of the transmitted beam) decreases as the propagation distance increases.
The receiving end of conventional free space optical communications typically requires a detector with a large active area, so as to capture as much of the transmitted beam as possible, thereby obtaining higher sensitivity. However, a large active area decreases the bandwidth of the detector. Moreover, there is added material and manufacturing cost associated with providing a large active area.
A single or group of lenses can be used to improve the sensitivity, by focusing the transmitted energy onto the active areas of the detector. However, the field of view of the detector is then limited by the extra optics. In addition, there are alignment issues associated with the use of lenses, particularly those systems involving multiple lens structures.
What is needed, therefore, are free space optical detection techniques that provide high speed, high sensitivity, large field of view, and relaxed tolerance on alignment.